1. Field of the Invention
This invention relates to an image reading apparatus, and more particularly to an image reading apparatus which is capable of performing an offset/gain adjustment in an efficient manner and with a simple structure.
2. Discussion of the Background
In a conventional image reading apparatus, an adjustment to an offset element and a gain of an image signal which is output from an image sensor and input to an A/D (analog-to-digital) converter has been performed on the image sensor side. More specifically, an image sensor used in the image reading apparatus normally includes a photoelectric conversion device for reading an image and generating an image signal in accordance with the image and an image signal correction circuit for adjusting an offset element and a gain of the image signal output from the photoelectric conversion device. Therefore, the offset/gain adjustment capability, such as by designed appropriate ranges for offsets and gains, for example, has been determined by the image sensors themselves in conventional image reading apparatus.
However, such an image sensor has a problem. When the image sensor is installed in an image reading apparatus, no matter how precisely the offset/gain adjustment capability is designed for an image sensor, the designed appropriate ranges for the offset/gain, in particular, may not necessarily fall in appropriate ranges for the image signal at an input stage of the A/D converter due to changes in the electrical environment.
Further, even when the offset/gain of the image signal at the input stage of the A/D converter falls in the respective appropriate ranges, the offset/gain may vary out of the designed appropriate ranges due to variations in power for the image reading apparatus, the light amount of a light source for the image sensor to read the image, and so forth caused in the image reading apparatus or for each installation environment. In such a case, it is necessary to take action, such as a replacement of the image sensor itself, for example, so as to repair the problem of the image reading apparatus, in which the offset/gain of the image signal falls outside the designed appropriate ranges.
Therefore, at present, there is no image reading apparatus which is capable of performing the offset/gain adjustment in an efficient manner and with a simple structure.